1. Field of the Invention
This invention is generally related to instrumentation for quantitatively determining the mineral content in ore and coal slurries, and has particular application in determining the P.sub.2 P.sub.5 content and CaO/P.sub.2 O.sub.5 ratio in phosphate ores.
2. Description of the Prior Art
One third of the world's phosphate production originates from the states of North Carolina and Florida in the United States. Phosphate reserves in North Carolina are concentrated primarily along the Atlantic Coastal Plain. They differ from Florida reserves in that they are generally deeper; however, the uniformity of the deposit tends to make extraction more efficient. Furthermore, the reagent dosages required to upgrade these reserves by froth flotation are usually less than required for Florida phosphate, while the phosphate lost during fine clay removal is about one-tenth of that associated with the Florida ores. However, North Carolina phosphate concentrates generally contain 30-31% P.sub.2 O.sub.5 as compared to the average Florida grade of around 33%. North Carolina concentrates also have a slightly higher CaO/P.sub.2 O.sub.5 ratio (calcium ratio) and, therefore, require additional sulfuric acid in the phosphoric acid production step.
A need exists for a suitable means of providing rapid analysis of phosphate grade and calcium ratio in process streams around the flotation circuit. Currently, phosphate grades are either determined in a qualitative manner by inspection (i.e., through the difference in color between the dark phosphate minerals and the light gangue minerals observed by an inspector watching the ore), or in a quantitative manner by wet chemical analysis. In the former case, the determination made is only as accurate as the personal judgement of inspector and this judgement can be affected by the lighting conditions in the plant. Furthermore, slight percentage differences in P.sub.2 O.sub.5 content are unlikely to be discernible. In the latter case, up to four hours of digestion and analysis may be required to generate an assay. Some common on-line analysis techniques such as X-ray fluorescence and neutron activation have been tested on phosphate ores, but they are generally inaccurate on materials of such low atomic number as calcium and phosphate. On-line nuclear magnetic resonance (NMR) has been used to determine P.sub.2 O.sub.5 content in slurries; however, this technique has not been successful in providing information on the calcium ratio (Cao/P.sub.2 O.sub.5).
O'Kane et al., "Preliminary Evaluation of a Computer Vision Sensor for Analysis of Phosphate Tailings", Control '90--Mineral and Metallurgical Processing, R. K. Rajamani and J. A. Herbst, eds. SME-AIME, pp. 143-149 (1990), discloses a small image analysis system which can identify different minerals in a sample of phosphate ore. The system requires the ore sample to be carefully spread out in front of a television camera whereby individual grains may be discerned. The minerals are then identified on the basis of the relative color intensities or reflectance from the individual mineral grains, and the quantities present are determined on the basis of the amount of area occupied by each grain in the complete image. Because this technique requires individual grains to be viewed, the quantity of sample which can be analyzed at any given time is small, and using small sample sizes may result in analysis errors because they may not be representative of the bulk stream. Furthermore, the technique is not suitable for on-line use.
On-line instrumentation would also be useful in monitoring coal slurries and other ores.
U.S. Pat. No. 4,797,550 to Nelson et al. discloses a fiber optic detector which is submerged in a coal slurry to monitor a coal separation process. In Nelson et al., light transmitted down a fiber bundle towards the slurry, is scattered by the slurry, and a portion of the light is reflected back to a fiber bundle and detected by a photosensor. The light intensity or hue of gray reflected by the slurry and detected by the photosensor is used to provide an indication of the ash content in the slurry. A disadvantage of the Nelson et al. system is that a small sample must be separated from the slurry and analyzed and, as pointed out above, small samples may not be representative of the bulk stream.